In many optical and electrooptical devices, like liquid crystal displays (LCDs), which comprise thin layers of an LC material provided on a substrate or between two substrates, it is often required to achieve uniform alignment of the LC material. Depending on the display mode it is for example desired to have planar alignment where the LC molecules are oriented with their long molecular axis parallel to the substrate, homeotropic alignment where the LC molecules are oriented with their long molecular axis perpendicular to the substrate, or tilted alignment where the LC molecules are oriented with their long molecular axis at an angle to the substrate.
The most conventional method described in prior art to achieve planar or tilted LC alignment is rubbing the substrate surface. This mechanical treatment has a big number of disadvantages, such as surface damaging, charging and dusting, complexity of patterning, and insufficient alignment uniformity on a microscopic level.
The intrinsic problems of LC rubbing alignment techniques stimulated the development of alternative alignment processes. Among them the so-called particle beam alignment methods show good promise for industrial applications. As the particles ions, neutral atoms, electrons, or mixtures thereof, in particular a plasma, can be used.
One can select three fundamental particle beam processes showing especially good promise for LC alignment:
1) surface etching,
2) sputtering deposition,
3) direct deposition.
The different processes mentioned above may occur simultaneously, but their efficiency depends on the energy of the particles. All three processes are discussed below and are schematically presented in FIG. 1.
In case of process 1) as shown in FIG. 1a, if the beam of accelerated (1) particles has an energy of 100 eV-10,000 eV, the so-called surface etching/milling process is preferred. In this case particles (1) bombarding the substrate (2) extract the substrate's atoms (3) and do thereby cause material ablation. This may be accompanied by braking chemical bonds and, in case of reactive gases, by plasma chemical reactions. This so-called surface etching process is mainly used for surface cleaning.
In case of process 2) as shown in FIG. 1b, if an accelerated beam of particles (1′) having an energy of 100 eV-10,000 eV is directed to any other substrate (4) (target), it causes material ablation from the target (4). The extracted particles (1) have a lower energy (<100 eV) and can be deposited on the desirable substrate (2) forming a film (3) thereon. This process is known as particle beam sputtering deposition.
Finally, in case of process 3) as shown in FIG. 1c, if a beam of particles (1) having very low energy (far less than 100 eV) is directed on the substrate (2), the particles have not enough energy to extract substrate's atoms. Instead, they may condense and react on the substrate forming a permanent film (3) thereon. This process is hereinafter referred to as direct (particle beam) deposition.
This classification includes only methods dealing with particle beams formed by ion and plasma beam sources. It does not include thermally initiated particle beams and associated methods like physical and chemical vapour deposition, which are much less convenient for LC technology, especially in case of coating large-area substrates.
To ensure uniform alignment of the LC molecules, the particle beam is usually directed obliquely to the alignment substrate. In this case, the surface of the modified film (in case of etching methods) or of the deposited film (in case of deposition methods) becomes anisotropic and thereby capable to align LCs. The induced surface anisotropy reveals itself in an anisotropy of relief and an anisotropy of molecular or intermolecular bonds.
Among the above-mentioned particle beam processes, surface etching 1) is the process most actively studied for LC alignment, and is disclosed for example in U.S. Pat. No. 4,153,529; P. Chaudhari, J. Lacey, S. A. Lien, and J. Speidell, Jpn J Appl Phys 37(1-2), L55-L56 (1998); P. Chaudhari et al, Nature 411, 56-59 (2001). In contrast to first attempts of etching alignment, in which particles of rather high energy (several keV) are used, in later experiments the energy is reduced to 0.1 keV. This allowed to treat only the very top layer of the alignment film so that surface deterioration is minimized. This technique provides low-pretilt alignment of good uniformity on the huge variety of organic and inorganic substrates.
By using plasma beam sources of linear construction, the etching technique is applied for the alignment treatment of large-area substrates used in modern LCD technology, as disclosed for example in WO2004/104682 A1. Recently, the etching process has also been proposed for the alignment of polymerizable LCs, also known as “reactive mesogens” (RMs), as disclosed in PCT/EP/2007/007078, and of solidified mesogens, as reported in O. Yaroshchuk et al., J. Soc. Inf. Display 16, 905 (2008).
The sputtering deposition method 2) is currently widely used as a laboratory tool for alignment of conventional LCs. As alignment films, SiOx coatings are usually applied. However, in spite of the prevalence of the sputtering method, it is not used as industrial alignment tool. The method is disclosed for example in U.S. Pat. No. 5,529,817, U.S. Pat. No. 5,529,817, U.S. Pat. No. 5,658,439. Matahiro and Taga, Thin Solid Films 185, 137-144 (1990) discloses a comparison of sputtering deposition and vapor deposition techniques. An insight in this alignment technology is also given in J. SID., 14/3, 257 (2006).
However, the etching and sputtering processes known in prior art do also have several disadvantages hindering their industrial application. For example, although it may achieve high alignment uniformity, the etching alignment process is still not industrialized because of alignment aging problems, i.e. alignment degradation with a storage time of LC cells. To overcome this problem, alignment materials and processing conditions should be thoroughly optimized, as reported in O. Yaroshchuk, R. Kravchuk, L. Dolgov, A. Dobrovolskyy, N. Klyui, E. Telesh, A. Khokhlov, J. Brill, N. Fruehauf, “Aging of LC alignment on plasma beam treated substrates: choice of alignment materials and liquid crystals”, Mol. Cryst. Liq. Cryst, 479, 111-120 (2007). In contrast to conventional LCs, the aging problem is less critical for RMs. The etching process can provide highly uniform planar alignment of RMs on a big number of substrates without any intermediate layers. However, this process is not very effective for homeotropic alignment. Also, RM alignment is frequently influenced by the properties of the substrate.
The sputtering deposition process supplements etching, because it is especially effective for (tilted and untilted) vertical alignment. However, problems do often occur in this method regarding alignment uniformity and alignment stability. Therefore, with regard to RMs, this alignment method is not systematically used.
Coatings obtained by direct deposition 3) from plasma have also been tested for LC alignment. The majority of these coatings are obtained by placing substrates directly in plasma discharge, as disclosed for example in J. C. Dubois, M. Gazard, and A. Zann, Appl. Phys. Letters, 24(7), 29738-40 (1974); R. Watanabe, T. Nakano, T. Satoh, H. Hatoh, and Y. Ohki, Jpn. J. Appl. Phys., 26(3), 373 (1987), and A. I. Vangonen, and E. A. Konshina. Mol. Cryst. Liq. Cryst., 304, 507 (1997). However, the resulting coatings reported in these documents are isotropic. Because of this, they cannot induce uniform tilted LC alignment without additional alignment action, which is usually rubbing.
Sprokel and Gibson in J. Electrochem. Soc., 124 (4), 557 (1977) and U.S. Pat. No. 4,261,650 describe deposition methods with a directed plasma flux under oblique incidence upon a substrate. The reactive particles carried by the gas stream condense on the substrate and form an alignment film, which provides uniform planar or homeotropic alignment depending on the type of coating formed and chemical composition of LC. The disadvantage of this method is that the particles originated from a “cold” plasma have low kinetic energy. This results in weak adhesion, low density and insufficient uniformity of the aligning films. Besides, this method is not suitable for the coating on large-area substrates.
U.S. Pat. No. 6,632,483 discloses a method of forming an amorphous film with an aligned atomic structure on a substrate, by bombarding the substrate with an ion beam generated from a carbon-containing gas. The gas is ionized in a discharge chamber to produce an ion beam comprising atoms and ions, which are then accelerated out of the ion source by applying acceleration voltages, and are directed to a substrate. The ion beam energy is 100-500 eV or higher. The film is reported to work as an alignment film in an LCD, however, no details or concrete examples of the achieved alignment are provided.
One aim of the present invention is to provide a method for aligning LCs and RMs without the need of rubbing, which provides uniform and stable alignment of the materials applied thereon, is easy to use in particular in mass production, and does not have the drawbacks of the prior art methods described above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that these aims can be achieved by providing a method as claimed in this invention. In particular, this invention relates to a principally different method of plasma beam deposition, which is easy in realization and provides films of high alignment quality. It can also be extended for the deposition on large-area substrates and plastic films. In addition to their LC alignment property, these films exhibit gas barrier, chemical and mechanical protective functions.